Composite materials which exhibit improved toughness due to the presence of an interlayer between filler particles and the matrix material are known. For example, Bixler et al (U.S. Pat. No. 3,471,439) teach composite materials having improved toughness which include a matrix, non-reinforcing filler particles distributed throughout the matrix and an interlayer prepared by the free radical polymerization of unsaturated reactants. In "Binding Rigid Polymers To Mineral Surfaces Through A Rubbery Interface", by Edwin P. Plueddemann, Proceedings of the 29th Annual Technical Conference, 1974, Reinforced Plastics/Composites Institutes, The Society of the Plastics Industry, Inc., Plueddemann teaches glass fiber reinforced resin matrix composites having a rubbery thermoplastic interlayer between the filler and matrix.